A small group of wealthy donors has already played a decisive role in the GOP presidential primary, analysts say, and could also help offset a powerful White House advantage in campaign fundraising for the general election in November.

To date, former Massachusetts Gov. Mitt Romney has been the main beneficiary of unlimited corporate giving. Restore Our Future, Inc., a super PAC backing Romney, picked up more than $30 million in contributions in 2011, mainly from financial industry executives and hedge-fund managers. Restore Our Future, Inc. spent $17.5 million to date, mainly on television ads attacking former House Speaker Newt Gingrich, according to the Center for Responsive Politics.

Winning Our Future, a super PAC supporting Mr. Gingrich, raised $2.08 million through 2011, nearly half from the daughter and son-in-law of Las Vegas casino billionaire Sheldon Adelson. In January, after the FEC’s reporting window, Mr. Adelson and his wife, Miriam, gave Winning Our Future two $5 million checks – and the capacity to fight on in South Carolina and Florida’s high-cost media markets.